Since many of heat generating devices such as CPU, a CD-ROM drive and a hard disk are incorporated in a personal computer, there is a need to dissipate generated heat so as to cool the devices in order to ensure normal operations over a long time. For this reason, various kinds of heat dissipating plates have been employed. However, especially, in notebook and portable personal computers, it is unavoidable that the heat generating devices are put inside a small case and a keyboard coexists in the case; many of the heat generating devices are resulted in being crammed in the narrow space, thereby leading to a desire for high heat dissipativity of a heat dissipating plate.
In the mean time, in the keyboard of a personal computer, since key caps with which fingers are put in direct touch are mounted on a base plate with a membrane or a pantograph structure interposed therebetween, and it is required that when a key is pressed down in typing, the base plate do not locally sink due to deformation, therefore a reinforcement plate is disposed on the lower side of the base plate to increase stiffness and cope with such a deformation. Such base and reinforcement plates each have a requirement for having a function as a heat dissipating member to prevent overheating inside the personal computer from occurring by diffusing heat generated from the devices therein in addition to a function as a strength member that supports the key caps and endure shocks of key strokes. Further, needless to say that the light weight and formability of the base and reinforcement plates are necessary.
For such reasons as described above, as material of the base and reinforcement plates, there have widely been adopted an Al--Mg based alloy including aluminum excellent in strength, good in thermal conductivity, and with features of light weight and good forming workability, and Mg in content of about 2.5 wt % especially to increase a mechanical strength.
While such an Al--Mg alloy has a sufficient strength and a good machinability including an excellent cutting property as compared with pure aluminum, the alloy has not been a satisfactory material in terms of heat dissipativity since a thermal conductivity thereof is lower by about 60%. Because of the poor heat dissipativity, the alloy has had a problem in a personal computer using the base and reinforcement plates made of the same since there arise overheating of CPU and other devices, and some of caps with high temperature are locally found in the entire arrangement.
Under such circumstances, the present inventor has proposed an Al--Si--Mg based alloy that seeks a heat dissipativity of pure aluminum, and a strength and machinability of the Al--Mg based alloy. Such a proposed Al--Si--Mg based alloy actually has a good thermal conductivity whereas the alloy is to some extent inferior to the Al--Mg based alloy in regard to a strength and thereby, the problems of overheating of CPU and other devices and caps of high temperature have almost been solved. However, part of the problems have still been left unsolved: Homogeneity in heat dissipation is insufficient and there are still locally found some key caps with high temperature, though, according to distances from the heat generating devices in a personal computer during its operation.